The difference between ‘woody stem’ and ‘soft stem’ is the existence of ‘tree ring’. By looking at the cross-section of a ‘woody stem’ plant, you can see the annual rings formed due to the growth of the stem(trunk). When you saw a bamboo stem, it’s empty inside. Therefore, bamboo is ‘grass’ and not ‘tree’. Bamboo belongs to the same group as paddy and millet.